This invention relates generally to semiconductor process equipment, and more particularly, to a method and systems for dispersing gas flow in a semiconductor reactor.
Semiconductor processing typically involves the formation of one or more layers on a semiconductor substrate. For example, silicon epitaxy, sometimes called epi, is a process in which one or more layers of single-crystal (monocrystalline) silicon are deposited on a monocrystalline silicon wafer.
FIG. 1 is a schematic representation of a semiconductor processing system 10 in accordance with the prior art. As shown in FIG. 1, system 10 included a susceptor 12 enclosed within a barrel reactor 14. Susceptor 12 was typically suspended from an assembly (not shown), which rotated susceptor 12 during processing. Susceptor 12 supported a plurality of substrates 16, typically monocrystalline silicon wafers.
During processing, substrates 16 were heated with an external radiation source such as tungsten halogen lamps, resistive heating elements and/or RF heaters (not shown). A process gas was introduced into reactor 14 through two injectors 18A, 18B mounted on a gas ring 20. The flow rate of the flow of process gas to injectors 18A, 18B was controlled by a mass flow controller 22 (MFC 22). Injectors 18A, 18B were coupled in parallel to MFC 22. The process gas reacted with heated substrates 16 resulting in the deposition of layers on substrates 16 as those skilled in the art understand. The spent process gas was then exhausted to exhaust 23.
As the art moves towards reduced feature size integrated circuits, it has become increasingly importance that the deposited layers on substrates have uniform thickness. One primary parameter which affects the thickness uniformity is the flow characteristics of the process gas into and through the reactor.
Referring again to FIG. 1, these flow characteristics were controlled to a large extent by injectors 18A, 18B through which the process gas was introduced in reactor 14. More particularly, injectors 18A, 18B aimed the jets of process gas so that the jets collided with each other at a point between susceptor 12 and reactor 14. The goal in aiming the jets was to eliminate any circumferential velocity components of the jets. The mixed jets flowed generally downwards over substrates 16 to the bottom of reactor 14 and to exhaust 23.
To obtained the desired thickness uniformity, injectors 18A, 18B were calibrated. Calibration was typically an iterative process in which a first layer was deposited on a first test substrate, the thickness uniformity of the first layer was measured, and injectors 18A, 18B were adjusted in an attempt to improve the thickness uniformity. A second layer was then deposited on a second test substrate, the thickness uniformity of the second layer was measured, and injectors 18A, 18B were again adjusted. This trial and error procedure was repeated until the desired thickness uniformity was obtained. Unavoidably, the iterative process used to calibrate injectors 18A, 18B was time consuming, labor intensive and generally unpredictable.
In addition to obtaining the desired thickness uniformity, it is also important to have abrupt transitions between layers. FIG. 2 is a graph of dopant concentration versus depth in a substrate 16 in accordance with the prior art. Referring to FIG. 2, formed on substrate 16 was a heavily doped layer L1 (hereinafter referred to as HD layer L1), a transition layer TL on top of HD layer L1, and a lightly doped layer L2 (hereinafter referred to as LD layer L2) on top of transition layer TL.
By way of example, HD layer L1 was a heavily doped P type silicon layer formed by supplying a process gas having a high P type dopant concentration. Conversely, LD layer L2 was lightly doped P type silicon layer formed by supplying a process gas having a low P type dopant concentration. Transition layer TL was formed as a result of the change from high to low of the P type dopant concentration of the process gas. As shown in FIG. 2, the dopant concentration of transition layer TL gradually changed from heavily doped HD at the bottom of transition layer TL to lightly doped LD at the top of transition layer TL.
As the art moves towards smaller high speed devices, it is important that the transition between layers be abrupt. In particular, referring to FIG. 2, it is important to reduce or eliminate transition layer TL between the top of HD layer L1 and the bottom of LD layer L2. However, use of system 10 (FIG. 1) inherently resulted in the formation of transition layer TL. This limitation of system 10 essentially eliminates the possibility of the use of barrel reactors for the next generation of integrated circuits. Yet, barrel reactors are relatively simple, reliable and cost effective to operate. Accordingly, the art needs a method and apparatus which allows realization of abrupt transitions between layers formed in a barrel reactor.
In accordance with the present invention, a semiconductor processing system includes a barrel reactor and a dispersion head within the barrel reactor. The dispersion head includes at least one distributor. During use, process gas is supplied to the dispersion head. The dispersion head is hollow (has an internal channel) so that the process gas flows through the channel of the dispersion head to the at least one distributor. The process gas flows through the at least one distributor and into the reactor. The process gas contacts substrates within the reactor thus forming a layer on the substrates. The spent process gas is then exhausted from the reactor.
Of importance, the process gas is dispersed by the dispersion head as the process gas enters the reactor. By dispersing the process gas, and supplying the dispersed process gas to the reactor, the flow characteristics of the process gas through the reactor is improved by reducing turbulence compared to the prior art. More particularly, use of the dispersion head reduces and/or eliminates turbulence and recirculation in the flow of the process gas through the reactor. Thus, the process gas travels through the reactor from the dispersion head to the exhaust in a uniform flow, i.e., in a curtain-like flow, without the turbulence and recirculation of the prior art.
Since the process gas flow is uniform through the reactor, the process gas uniformly contacts the substrates. Accordingly, use of the dispersion head results in the formation of layers on the substrates having excellent thickness uniformity. Further, since the dispersion head disperses the process gas in a repeatable and predefined manner, calibration of the dispersion head is avoided. This is in contrast to the prior art where the injectors had to be calibrated for each reactor and also had to be recalibrated when process parameters, e.g., the flow rate of flow of process gas, were changed. Thus, use of a dispersion head in accordance with the present invention is less time consuming, is less labor intensive and is more reliable than use of the injectors of the prior art.
In addition, use of the dispersion head allows realization of an abrupt transition between layers formed on the substrates. This is because when the process gas is changed to have a new composition, e.g., from a high dopant concentration process gas to a low dopant concentration process gas, the new process gas travels in a uniform flow through the reactor similar to a curtain falling. As the bottom of this curtain passes the substrates, the process gas contacting the substrates abruptly changes to have the new dopant concentration. Thus, an abrupt transition occurs between the layer formed from the process gas having the original gas composition and the new layer formed from the process gas having the new gas composition. Accordingly, use of the dispersion head enables formation of substrates having abrupt transitions between layers using a relatively simple, reliable and cost effective barrel reactor.
In one embodiment, a system includes a barrel reactor and a plurality of dispersion heads extending into the barrel reactor. Each dispersion head of the plurality of dispersion heads is a different tube of a plurality of tubes. These tubes extend through ports of a seal plate or, alternatively, through ports of an inverted barrel reactor.
Also in accordance with a present invention, a method includes dispersing a process gas and supplying the dispersed process gas to a barrel reactor. The process gas travels through the barrel reactor in a uniform flow. This results in the formation of layers on substrates in the reactor having excellent thickness uniformity. This also results in abrupt transitions between layers formed on the substrates.
In one embodiment, a method includes placing a substrate in a reactor. The method further includes supplying a flow of a carrier gas to a dispersion head within the reactor and supplying a flow of a first process gas through injectors to the reactor. The first process gas contacts the substrate thus forming a layer on the substrate. Upon shutting off the flow of the first process gas, the carrier gas uniformly displaces the first process gas thus abruptly terminating the formation of the layer on the substrate.
These and other features and advantages of the present invention will be more readily apparent from the detailed description set forth below taken in conjunction with the accompanying drawings.